An inkjet recording method is a printing method wherein printing is performed by ejecting small droplets of an ink composition and attaching them to a recording medium such as paper. The inkjet recording method has characteristics that images having a high resolution and a high quality can be printed in a high speed. The ink composition for use in the inkjet recording method generally contains an aqueous solvent as a main component and further a coloring component and a wetting agent such as glycerin for the purpose of preventing clogging.
On the other hand, in the case where printing is performed on printing media such as some kinds of paper and fabric into which an aqueous ink composition is difficult to permeate or plates and films made of raw materials such as metals or plastics, e.g., phenol, melamine, vinyl chloride, acryl, polycarbonate and the like resins, it is required for an ink composition or a reaction liquid to contain a component which enables a coloring material to be fixed onto the recording media stably. In particular, recently, the recording method has been used for plastic cards, color filters, print-circuit boards, and the like.
For such a requirement, there have been disclosed an ultraviolet-curable inkjet ink containing a coloring material, an ultraviolet curing agent (polymerizable compound), a (photo-)polymerization, initiator, and the like (see, e.g., Patent Document 1). It is described that according to the ink, blurring of the ink on recording media can be prevented and image quality can be improved.
Such an ultraviolet-curable ink is for a process in which the ink is attached onto a recording medium and the resulting medium is then irradiated with an ultraviolet ray to cure the ink. Heretofore, as a means for curing such ultraviolet-curable ink, there has been used a low-pressure, high-pressure, or ultrahigh-pressure mercury lamp, a xenon lamp, a metal halide lamp, or the like.
Further, there is a case that an ultraviolet-curable ink contains a coloring material such as pigment. Coloring materials such as pigments have strong scattering/absorption also in an ultraviolet region in many cases and hence the irradiated ultraviolet ray is not effectively utilized, so that ultraviolet-curable inks containing coloring materials are apt to cause a curing defect.
In conventional art, in order to prevent the curing defect of the ultraviolet-curable ink as described above, there exist methods wherein irradiation of a large excess of ultraviolet ray is carried out by means of a mercury lamp or the like, or curing with an ultraviolet ray and heat treatment are used in combination. However, these methods are not preferred because of high facility costs and also there are problems that irradiation by a mercury lamp requires a cooling device and the life of the light source is short, so that the methods are not easy to use. Further, in these methods, since heat is applied to the recording media, there is also a problem that usable recording media are limited.
Recently, with the development of semiconductor light-emitting devices capable of emitting only an ultraviolet light having a specific wavelength range at a high intensity and requiring a small power consumption, such as ultraviolet light-emitting diodes (ultraviolet LED), adoption of these semiconductor light-emitting devices as light source enables elimination of impartment of heat damage to the recording media induced by the combined use of heat treatment as well as energy saving and miniaturization and weight saving of the facilities (see, e.g., Patent Document 2).
Patent Document 1: U.S. Pat. No. 5,623,001
Patent Document 2: JP-A-2003-326591
However, unlike the ultraviolet light emission consisting of a conventional continuous spectrum emitted by a high-pressure mercury lamp, a xenon lamp or the like, light emission induced by an LSD is a single peak and a spread of the peak width in the wavelength direction is very small (±5 to 10 μm as half-band width). Therefore, it is still very difficult to cure the ink in the case where a reflection/absorption peak of a coloring material such as a pigment to be used overlaps with an ultraviolet peak to be irradiated.